The present invention relates generally to testing apparatus, and more particularly, to an apparatus for destructive testing of the strength of parts subjected to crimping or crush forces in use.
Molded plastic parts have attained wide spread use in many assemblies. With the ever-improving strength of plastic materials now available, plastic parts are used as structural components that must withstand significant force and or loads. The lighter weight and resistance to deterioration such as corrosion and rusting of plastic materials have made plastic parts preferable to metal parts in a variety of applications. For example, in the automotive industry, suspension systems can include hollow plastic parts to which a rubber or elastomeric boot is affixed by a clamping ring to effect a seal tight against internal hydraulic pressures. The force exerted on the plastic part during crimping of the clamping ring can be extremely high, particularly when steel rings are used. The plastic part must withstand such force.
When the plastic part is relatively thick, injection or fill speed can affect several physical qualities in the final product. Faster fill or injection speeds tend to provide a smoother surface finish, but can result in an increase in the presence of voids in the final product. Voids can lead to a reduction of strength in the part. Slower fill times tend to result in fewer voids, but a less smooth surface finish. Moreover, a generally smooth surface finish is often required to seal Brings for system pressure. Therefore, achieving a desired surface finish and strength in the final product can be a balance between competing needs.
In relatively thick parts it is virtually impossible to have a completely void free structure. However, the mere presence of voids does not necessarily mean the part is insufficiently strong for the intended application. Voids may be sufficiently small and sufficiently infrequent that the part will have adequate strength for its intended purpose. Further, the voids may be present in locations of relative insignificance. Therefore, while it is feasible to check all parts by x-ray or other techniques to determine the presence of voids, such testing is expensive, greatly increasing the manufacturing cost for the injection molded part, and the test is not a true indicator of the strength of the part.
When plastic parts are manufactured in batches from a single batch of materials, with each part formed under substantially similar conditions, all of the parts thus formed tend to exhibit substantially similar characteristics. This can be used advantageously for quality control testing in that a representative sample of parts from each batch can be subjected to testing, with the results thereof applied to all parts in the batch. If the tested parts pass quality control testing, all parts in the batch are deemed to have passed. On the other hand, if the representative sample fails testing, the entire batch from which the sample was selected is deemed to have failed, and will be scrapped. This approach to quality control testing, wherein only a sample and not all parts are tested, makes it possible to use destructive testing techniques on the samples. In destructive testing, the part is subjected to conditions of concern until the part fails. In this way, it can be determined not only if the established standards are met, but also if the standards are exceeded, and by how much. The manufacturing conditions can then be adjusted, if trends arc perceived from successive batches.
It is desirable to have a safe, efficient and rapid test method and apparatus for testing a representative sample of parts from manufacturing processes, which will determine the crush strength or crimp strength of the part at the location of significance on the part, to determine if the part meets standards or greatly exceeds standards. It is desirable that an apparatus therefore be adjustable for testing a variety of parts of different sizes.
The present invention provides a destructive testing module that operates efficiently and easily to perform crush strength testing at the critical area of a part, and that is adaptable for use with parts of different diameters.
In one aspect thereof, the invention provides a test module for destructive testing of a part. Fingers are arranged to define a center opening of a size to receive the part to be tested. Each finger has an inner end to engage the part, and an outer end. Each outer end has a cam follower engaged with a separate cam for each cam follower. The cams are adapted for operation in unison to drive the fingers in unison toward the center opening. A load cell is operatively connected to at least one finger, to measure load exerted on the part in the center opening. A reporting means is connected to the load cell for communicating the load sensed by the load cell.
In another aspect thereof, the invention provides a crush strength testing module with a ring having an inner face defining a plurality of cams, and a plurality of radially extending fingers. Each finger has a cam follower engaged against one of the cams. The inner ends of the fingers collectively define an opening therebetween. The cams and the fingers are adapted and arranged for simultaneous movement of the fingers radially inwardly in the ring, for decreasing the size of the opening upon rotation of the ring. A load cell is operatively connected to one of the fingers. A reporting means is connected to the load cell for reporting load sensed by the load cell. A drive means is connected to the ring, for rotating the ring and moving the fingers.
In still another aspect thereof, the invention provides a method for testing crush strength of a part, the method including said steps providing a plurality of fingers disposed radially about a central opening, each finger being movable radially toward the opening; providing cams and a drive means for moving the fingers in unison, and sensing means for ascertaining load force exerted on a part in the opening; positioning a part in said opening; moving the fingers against the part in unison, with sufficient force to cause structural failure of the part; and determining the load exerted against the part to cause the structural failure thereof.
An advantage of the present invention is providing a destructive testing apparatus that closely approximates the force applied to a part in regular use, and determines the actual crush resistance of the critical area of the tested part.
Another advantage of the present invention is providing a test module that is easy to use and adaptable for use with parts of different diameters.
Still another advantage of the present invention is providing an enclosed testing module that contains the broken pieces from a tested part, and thereby protects an operator of the module.